Coöperatieve binding van transcriptieregulatoren

Transcriptional regulators are proteins that recognize and bind to short sequences of DNA known as cis-regulatory sequences. Because these sequences are usually less than ten nucleotides long, the chances of the same sequence randomly occurring in the genome are very high.

Many regulators form a dimer pair in order to limit binding to random sequences. A dimer can bind sequences longer than ten nucleotides making it less likely that the sequence will be randomly present in the genome and increasing the binding specificity.

These regulatory dimers can either be a homodimer made of the same types of monomer or a heterodimer with different types of monomers.  Because the pairs can be composed of different monomers, the various combinations allow binding to different sequences without needing new types of proteins.

When not bound to DNA, cooperative regulators exist as monomers that sometimes form dimers through weak, non-covalent interactions; however, these structures form tightly associated dimers when bound to DNA due to cooperative binding.

Cooperative binding is a phenomenon where the binding of a monomer to the cis-regulatory sequence increases the likelihood of a second regulator binding due to structural changes. This allows a second regulator to bind tightly to the other side of the binding site and form a dimer with the first.

This means that most of the time either all instances of specific cis-regulatory sequences have a regulator bound or none of them do. 

For many genes, the DNA is wrapped tightly around histone proteins preventing transcriptional regulators from accessing the cis-regulatory sequences. However, the loosely bound end of the DNA allows some room for binding. The binding of a single regulator at this site can help to unwind the structure, thereby allowing the other regulators to bind.